Peripheral vascular array

ABSTRACT

An interface that couples a surface coil array having N receiving elements to an NMR scanner having M preamplifier inputs. The interface includes an RF switch array and a transmit/receive bias circuit. The RF switch array and the transmit/receive bias circuit are controlled by a control logic circuit. In response to a predetermined input, the control logic circuit causes a predetermined subset of the N receiving elements to be couple to the preamplifier inputs of the NMR scanner. Preferably, N may be larger than M.

BACKGROUND OF THE INVENTION

The present invention relates to nuclear magnetic resonance (“NMR”)imaging and, more particularly, to methods and apparatus for imaging theperipheral vasculature.

Initially, NMR imaging systems utilized receiver coils which surroundedthe entire sample (for example a human patient) that was to be imaged.These remote coils had the advantage that the sensitivity was, to afirst approximation, substantially constant over the entire region beingimaged. While this uniformity in sensitivity is not strictlycharacteristic of such remote coils, the sensitivity is substantiallyconstant to a sufficient degree that most reconstruction techniquesassume a constant coil sensitivity. Because of their large size theremote coils suffer from a relative insensitivity to individual spins.

For certain applications, a surface coil is preferable to a remote coil.Surface coils can be made much smaller in geometry than remote coils andfor medical diagnostic use can be applied near, on, or inside the bodyof a patient. This is especially important where attention is beingdirected to imaging a small region within the patient, rather than anentire anatomical cross section. The use of a surface coil also reducesthe noise contribution from electrical losses in the body, with respectto a corresponding remote coil, while maximizing the desired signal. NMRimaging systems thus typically use a small surface coil for localizedhigh-resolution imaging.

A disadvantage of the surface coil, however, is its limited field ofview. A single surface coil can only effectively image that region ofthe sample having lateral dimensions comparable to the surface coildiameter. Therefore, the surface coil necessarily restricts the field ofview and inevitably leads to a tradeoff between resolution and field ofview. The size of the surface coil is constrained by the intrinsicsignal to noise ratio of the coil. Generally, larger coils inducegreater patient sample losses and therefore have a larger noisecomponent, while smaller coils have lower noise but in turn restrict thefield of view to a smaller region.

One technique for extending the field-of-view limitation of a singlesurface coil is described in U.S. Pat. No. 4,825,162 to Roemer et al.Roemer et al. describes a set of surface coils arrayed with overlappingfields of view. Each of the surface coils is positioned so as to havesubstantially no interaction with all adjacent surface coils. Adifferent NMR response signal is received at each different one of thesurface coils from an associated portion of the sample enclosed withinan imaging volume defined by the array. Each different NMR responsesignal is used to construct a different one of a like plurality of NMRimages of the sample, with the plurality of different images then beingcombined to produce a single composite NMR image. Roemer et al.describes a four-coil array for imaging the human spine.

While an increased number of surface coils may be used to increase thefield of view, NMR system scanners typically have a limited number ofpreamplifier input. The number of preamplifier inputs is therefore adesign limitation in the design of phased array surface coils. Adisadvantage of known phased array surface coils, therefore, is that thesurface coil array may include only as many coils as can be directlyconnected to the preamplifiers of the system scanner.

One technique for constructing images of areas of greater size from thelimited filed of view of known surface coil combinations is to move thesurface coils after successive scans. This technique however, requiresexcessive scan room intervention. That is, after each scan, a technicianenters the scan room to physically re-position the coils. This mayincrease examination time and increase the likelihood of a patientrejecting the procedure.

It would be desirable to obtain increased field of view without scanroom intervention.

It would also be desirable to have an improved phased array surface coilfor providing a large field of view. It is further desirable to utilizea greater number of surface coils in the array.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, a coilinterface is provided. The coil interface includes N coils for sensingimage signals and a number of switches connected to the N coils. Thecoil interface also includes circuitry for selecting a group of the Ncoils. The selection may be made by enabling a selected group of thenumber of switches in response to a group selector input. The coilinterface further includes a number, M, of outputs to an NMR scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic design of a system for receiving an NMR responsesignal in accordance with a preferred embodiment of the presentinvention.

FIGS. 2A and 2B are schematic representations of a peripheral vasculararray that is operable with the system of FIG. 1.

FIG. 3 illustrates an embodiment of a housing for the peripheralvascular array shown in FIGS. 2A and 2B.

FIGS. 4A, 4B and 4C schematically illustrate the capability of theperipheral vascular array housing shown in FIG. 3 to accommodate avariety of body types.

FIGS. 5 through 16 are electrical schematic diagram of the surface coilsin the peripheral vascular array shown in FIGS. 2A and 2B.

FIG. 17 is a block diagram of an NMR scanner and a 20-coil surface coilarray that uses an interface in accordance with a preferred embodimentof the present invention.

FIG. 18 is a coil group table showing groups of surface coils, a modeswitch setting, surface coils selected by a particular group andcomments regarding an image obtained using the selected group of surfacecoils.

FIG. 19 is an electrical schematic of the T/R driver shown in FIG. 17.

FIG. 20 is an electrical schematic of the RF switch array shown in FIG.17.

FIG. 21A through 21C are electrical schematics of the RF switches shownin FIG. 20.

FIGS. 22A and 22B are electrical schematics of a preferredimplementation of the RF switch array shown in FIG. 20.

FIG. 23A illustrates a programmable logic device in a preferredimplementation of the control logic shown in FIG. 17.

FIGS. 23B and 23C are state tables for the control logic shown in FIG.17.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of a system for receiving an NMR responsesignal in accordance with a preferred embodiment of the presentinvention. The system includes a surface coil array 10 that is connectedby an interface 20 to an NMR scanner 30. The surface coil array 10includes a number, N, of surface coils 12. Each of the surface coils 12is electrically connected through a transmit/receive (“T/R”) biascircuit 21 to an RF switch/combiner 22 in the interface 20. The RFswitch/combiner 22 has a plurality of outputs 23 that are connected to aplurality of receiver preamplifiers 32 in the NMR scanner 30.

The interface 20 also includes a control logic circuit 24, which iscoupled to and controls the RF switch/combiner 22 and the T/R bias 21.The control logic circuit 24 has three inputs. The first input is a DCpower input 26, which is provided by a power supply 34 in the NMRscanner 30. The second input is a coil select input 28. The third inputis a mode select input 25. The control logic circuit 24 selectivelyactivates a predetermined arrangement of surface coils 12 in accordancewith the coil select input 28 and the mode select input 25.

As shown in FIG. 1, the coil select input 28 originates from the NMRscanner 30. However, the coil select input 28 may alternatively beprovided to the control logic circuit 24 from another source that isexternal to the interface 20, as illustrated by the dashed coil selectinput 29 in FIG. 1. The coil select input 28 is preferably a DC controlsignal. Blocking capacitors may therefore be used at the outputs 23 ofthe RF switch/combiner 22 to block DC from the RF switch 22. In asimilar manner, RF chokes may be used on the coil select input 28 toprevent RF signals from reaching the control logic circuit 24. Thedesign of DC blocks and RF chokes are well known to those of ordinaryskill in the art. When the coil select input 28 originates from the NMRscanner 30, it may be superimposed on some or all of the outputs 23. Asshown in FIG. 1, four outputs 23 are used, such that the coil selectsignal 28 may be treated as a 4-bit word.

In addition, as shown in FIG. 1, the DC power input 26 is provided bythe power supply 34 in the NMR scanner 30. However, the DC power input26 may alternatively be provided by a power supply, such as a battery,contained within or connected to the interface 20. Preferably, thebattery is constructed of materials that are not responsive to and donot adversely effect the magnetic fields in the NMR examination room.For example, the battery may be a rechargeable lead-acid battery with agel electrolyte and a plastic housing, such as the Gel Cell™ batteriesthat are commercially available from Globe. As a still furtheralternative, the DC power input 26 may be provided by any DC source thatis external to the scanner 30. These alternatives are particularlyappropriate when the scanner 30 either does not provide a DC powersupply output or provides a DC power supply output that is insufficientto power the interface 20 and the array 10.

During imaging, the surface coils 12 that are activated to the receivestate produce RF signals that are coupled to the RF switch/combiner 22.The outputs 23 of the RF switch/combiner 22 are then coupled topreamplifiers 32 in the NMR scanner 30. The operation of the interface20 is described in greater detail below with reference to FIGS. 17through 22.

The scanner 30 includes a predetermined number, M, of the receiverpreamplifiers 32. In accordance with a preferred embodiment of thepresent invention, the interface 20, as described below, allows asurface coil array 10 having a greater number, N, of surface coils 12than the number, M, of receiver preamplifiers 32, to be connected to thescanner 30 (i.e. N≧M).

In accordance with a preferred embodiment, the array 10 is areceive-only, phased array surface coil and the NMR scanner 30 iscapable of operating in a phased array receiving mode. Such NMR scannersare commercially available. For example, the Signa™ family of magneticresonance imaging systems, equipped with phased array capability, areavailable from GE Medical Systems, Inc. of Waukesha, Wis. These scannersare designed to accept up to eight preamplifier inputs (M=8).

In accordance with a preferred embodiment of the present invention, thesurface coil array 10 is a peripheral vascular array. The peripheralvascular array is useful for studies relating to peripheral vasculardisease. These studies may include deep vein thrombosis screening,aortic runoff studies, distal vessel patency, the determination of theexistence location, length, and severity of stenoses, and the search forpatent distal vessels that are suitable for bypass grafts. Theperipheral vascular array is therefore preferably capable of imagingvasculature from the area of the kidneys and descending through thelower extremities to the feet. Because of this large field of view, theperipheral vascular coil may also be useful for applications involvingsoft tissue imaging, such as screening for metastic disease, and longbone imaging.

FIGS. 2A and 2B show a schematic representation of a peripheral vasculararray 40 in accordance with a preferred embodiment of the presentinvention. In FIG. 2A, an arrangement of the surface coils in the array40 is illustrated. The peripheral vascular array 40 includes tenposterior coils, 42 through 51, and ten anterior coils, 52 through 61.Each of the coils 42 through 61 is a surface coil that receives signalsfrom hydrogen protons during NMR imaging.

Referring again to FIG. 2A, surface coils 42, 43, 52 and 53 are singleloop coils. Surface coils 44 a, 44 b and 45 a, 45 b are counter-rotatingloops. Surface coils 54 a, 54 b and 55 a, 55 b are co-rotating loops.The surface coils in the lower leg section, surface coils 46-51 and56-61, are single loops when operated in unilateral mode, as describedbelow. In bilateral mode, which is also described below, surface coilpairs (46, 47), (48, 49) and (60, 61) are combined as co-rotating loopsand surface coil pairs (50, 51), (56, 57) and (58, 59) are combined ascounter-rotating loops.

FIG. 2B illustrates how the surface coils shown in FIG. 2A may bearranged about the patient to obtain images of the vascular structuresof the abdomen, pelvis and lower limbs. In particular, the peripheralvascular array 40 may obtain images of the vascular structures from therenal arteries through the feet without moving the patient or the array40. Thus, the peripheral vascular array 40 advantageously allows a largeregion of the patient to be imaged without requiring scan roomintervention by an operator. This may decrease examination times andminimize the likelihood of patient rejection.

As shown in FIGS. 2A and 2B, with the exception of the surface coils 42,43 and 52, 53, where an anteriorly located surface coil overlies aposteriorly located coil, one of the coils is of the co-rotating typeand the other is of the counter-rotating type. In addition, whereadjacent surface coils in the array 40 may significantly overlap, suchthe surface coil 45 a, 45 b and the surface coil pair 46, 47, one of thecoils is preferable of the co-rotating type (e.g. pair 46, 47) and theother is of the counter-rotating type (45 a, 45 b). This alternationbetween co-rotating and counter-rotating structures provides the benefitof improving the isolation between the coils, whose intrinsic isolationis then maintained even if the vertical spacing between the coilschanges, the superior/inferior offset between opposing coils changes, orthe superior/inferior offset between adjacent coils is adjusted. Thesuperior/inferior offset between adjacent coils may be adjusted, forexample, by telescoping the surface coils 45 through 51 and 56 through61 toward or away from the surface coils 45 and 55.

FIG. 3 shows an exploded view of a housing 62 for the peripheralvascular array 40. The housing 62 is constructed to position the coilsof the peripheral vascular array 40 as shown in FIG. 2B. The housing 62includes a tray 64 that is constructed to support the legs of thepatient. The tray 64 has a recess 66 at its distal end. At the oppositeend of the tray 64 from the recess 66, an incline 68 is formed in theupper surface of the tray 64. A lumbar support 70 extends from the tray64 at an edge 72 adjacent to the incline 68. The lumbar support 70includes a support surface 74 and a positioning member 76. Thepositioning member 76 fixes the relative portion of the lumbar support70 and the tray 64. In the alternative, the positioning member 76 mayallow the lumbar support 70 to be extended from or drawn closer to thetray 64 in order to accommodate patients of varying size. The positionmember 76 extends into the tray 64 at the edge 72.

The housing 62 also includes a leg support structure 78. A top portion80 of the leg support 78 is attached to a bottom portion 82 by a coupler84. The coupler 84 preferably allows the position of the top portion 80to vary with respect to the bottom portion 82. The leg support 78 isslidably mounted within the recess 66 in the tray 64 so that the uppersurface of the bottom portion 82 is flush with the upper surface of thetray 64. Because the leg support 78 is slidably mounted, it may be movedto accommodate variations in patient size.

The housing 62 further includes a cover 86. A first end 88 of the cover86 is shaped to fit over the top portion 80 of the leg support 78. Inthis manner, the cover 86 may slide over the top of the leg support 78when the leg support 78 is moved along the recess 66. A flexibleextension 90 protrudes from a second end 92 of the cover 86.

The location of the surface coils within the embodiment of the housing62 that is shown in FIG. 3 will now be described with reference to FIGS.2A and 2B. The tray 64 includes posterior surface coils 44 and 45, withthe surface coil 44 being substantially located below the surface of theincline 68 and the coil 45 being located below the surface of the tray64, extending substantially into the recess 66. The lumbar support 70houses posterior coils 42 and 43. The bottom portion 82 of the legsupport 78 houses posterior coils 46, 47, 48, 49, 50 and 51, with theeven-numbered coils being located beneath the region of the patientslower right leg and foot, and the odd-numbered coils being locatedbeneath the region of the patients lower left leg and foot. The topportion 82 of the leg support 78 houses anterior coils 56, 57, 58, 59,60 and 61 in the same manner. Finally the cover 86 houses anterior coils52, 53, 54 and 55.

In accordance with a preferred embodiment of the present invention, thehousing 62 allows the peripheral vascular array 40 to accommodate a widevariety of body styles, including variations in height and weight. Forexample, as shown in FIG. 2B, only posterior coils 44 and 45 are fixedin position with respect to the tray 64. The locations of the lumbarsupport 70, leg support 78, and cover 86 may all vary with respect tothe tray 64.

In addition, the housing 62, and more particularly the tray 64 andlumbar support 70, is preferably designed to align the peripheralvasculature of the patient into a horizontal plane. For example, thelumbar support 70 is located to raise the plane containing the renalarteries with respect to the patient's pelvis, where the external illiacis situated, so that the renal arteries and the external illiac becomegenerally coplanar. In a similar manner, the incline 68 positions thepatient's thighs so as to place the femoral arteries in the plane of theexternal illiac and the renal arteries. The leg support 78 and the tray64 then position the popliteal and tibial arteries into approximatelythe same plane. By then limiting the NMR imaging process to the regionabout the horizontal plane containing the vasculature of interest, theexamination time may be reduced.

Referring again to FIG. 2B, it is evident that the cover 86 is supportedat one end by the leg support 78 and at the opposite end by the body ofthe patient. The flexible extension 90, therefore allows the anteriorcoils 52, 53, 54 and 55 within the cover 86 to remain as close to thepatient as possible. In accordance with an alternative embodiment of thepresent invention, one or more loaded arms (not shown) may be used tosupport an anterior portion of the housing 62 away from the patient.Such loaded arm techniques are well known to those skilled in the art.

FIGS. 4A, 4B and 4C schematically illustrate the flexibility of theperipheral vascular array 40 in accommodating a variety of body types.As shown in FIGS. 4A through 4C the peripheral vascular array 40 isadvantageously able to maintain imaging coverage from the renal arteriesthrough the feet whether the patient is of relatively small stature,such as in FIG. 4A, or large stature, such as in FIG. 4C. This isadvantageous because patients afflicted with peripheral vascular diseaseare frequently significantly larger or smaller than the average person.

The top portion 80 of the leg support 78 and the cover 86 are preferablyconstructed in a lattice type framework. This reduces the weight of thecoil, making it easier for the technician to use while at the same timeimproving patient comfort by allowing air to flow around the patient toenhance cooling. In addition, the housing 62 allows the patient's armsto remain unrestricted, thereby reducing claustrophobic reactions thatare sometimes experienced by patients who are subjected to the closeconfines of NMR scanners. Moreover, once the peripheral vascular array40 is adjusted to accommodate the size of the patient, no patient orcoil movement is required to complete the examination. This reducesexamination times and increases patient comfort.

The surface coils 42-61 are typically formed from copper traces having athickness of 0.0028″ and a width of 0.5″. Copper bars or tubing mayalternatively be used as coil conductors. The peripheral vascular array40 preferably contains a practical minimum of conductive materials. Thiswill aid in the reduction of eddy currents at the frequenciescorresponding to NMR gradient coil wave forms, thus minimizing thepossibility of artifacts. In addition, the peripheral vascular array 40preferably contains a practical minimum of ferro-magnetic materials tominimize the interaction of the array 40 with the B₀ main magnetic fieldof the host NMR system.

The tray 64 is preferably made from ABS using a vacuum forming process.The surface coils 44 and 45 are then adhered to the bottom of the uppersurface of the tray 64. The bottom portion 82 of the leg support 78 ispreferably constructed using a low-pressure polyurethane resin to encasethe surface coils. Flexible areas of the housing 62, such as theflexible extension 90, are formed by sandwiching the surface coilsbetween foam and then encasing the foam in fabric cover.

FIGS. 5 through 16 are electrical schematic diagrams of the surfacecoils 42 through 61. Each of the surface coils 42 through 61 preferablyincludes a PIN diode, D, for switching the surface coils 42 through 61between the receive state and an active disabled state. This providesthe advantage of decreasing undesirable coil interaction that may reduceimage quality, particularly during unilateral imaging. As is known inthe art, the surface coils are preferably actively disabled by PIN diodeswitches during the RF transmit state. In addition, FIGS. 5 through 16show passive blocking networks 96. The passive blocking networks 96assist the PIN diode switches in disabling the surface coils 42-61during the RF transmit state.

Furthermore, FIGS. 7 through 16 show implementations of networks 94,including component values, for isolating adjacent coils to furtherimprove image quality by reducing shading and aliasing artifacts. Thenetworks 94 in FIGS. 7 through 16 also perform the matching andswitching functions. As shown in FIGS. 5 and 6, the mutual inductancebetween surface coils 42 and 43 and between surface coils 52 and 53 isreduced by overlapping adjacent coils in a manner that is known to thoseskilled in the art although isolation networks may alternatively beused.

Referring now to FIG. 5 , an electrical schematic diagram for thesurface coils 42 and 43 is provided. Each surface coil 42 and 43includes a passive blocking network 96, loop capacitances, and an inputnetwork 98. The input network 98 includes the PIN diode, D₁, blockingand matching elements, and a 50-Ohm lattice balun 100. The balun 100,which is also shown in FIGS. 6 through 16 (although with differingcomponent values), suppresses common-mode currents. Component values forthe elements shown in FIG. 5 are as follows: Loop Components Balun 100Input Network 98 C₁, C₅ = 1-16 pf L₂, L₃ = 142 nh C1 = 91 pf C₂ = 75 pfC₃, C₄ = 51 pf C2 = 100 pf C₆ = 91 pf D1 = UM 9415 PIN diode C₃, C₄ = 82pf L1 = 92 nh C₇, C₈ = 82 pf C₉, C₁₀ = 82 pf Diodes = Unitrode Diodes

FIG. 6 is an electrical schematic diagram for surface coils 52 and 53.Component values for the elements shown in FIG. 6 are as follows: LoopComponents Balun 100 Input Network 98 C₁, C₂ = 51 pf L₂, L₃ = 142 nh C1= 56 pf C₃, C₅ = 47 pf C₃, C₄ = 51 ph C2 = 100 pf C₄, C₆ = 1-16 pf D1 =UM 9415 PIN diode L1 = 92 nh

FIG. 7 is an electrical schematic diagram for surface coil 44. Inaddition to passive blocking networks 96, loop capacitances and a 50-Ohmlattice balun 100, the surface coil 44 includes a network 94 forisolating the counter-rotating loops of the surface coil 44. Componentvalues for the elements shown in FIG. 7 are as follows: Network(Correction, Loop Components Balun 100 Decoupling & Match) 94 C₃ = 39pf + 1-16 pf C = 51 pf C1 = 75 pf C₄ = 39 pf + 1-16 pf L = 142 nh L1 =88 nh C₁ = 56 pf C2 = 75 pf C₂ = 47 pf L2 = 88 nh C₅ = 47 pf L3 = 4.7 uhC₆ = 56 pf C3 = .01 uf C4 = .01 uf C5 = .01 uf C6 = .01 uf Lcomp = 114mh Diodes = UM 9415 PIN Diode

FIG. 8 is an electrical schematic diagram for surface coil 54. A network94 is included for isolating the co-rotating loops of the surface coil54. Component values for the elements shown in FIG. 8 are as follows:Network (Correction, Loop Components Balun 100 Decoupling & Match) 94 C₁= 47 pf C = 51 pf C1 = 56 pf C₂ = 47 pf L = 142 nh L1 = 110 nh C₃ = 47pf C2 = 56 pf C₄ = 47 pf L2 = 110 nh L3 = 4.7 uh Ccomp = 43 pf C3 = N/AC4 = N/A C5 = .01 uf C6 = .01 uf Ctune = 39 pf + 1-16 pf Diodes = UM9415 PIN Diode

FIG. 9 is an electrical schematic diagram for surface coil 45, whichincludes two counter-rotating loops 45 a and 45 b. A network 94 isincluded for isolating the loops. Component values for the elementsshown in FIG. 9 are as follows: Network (Correction, Loop ComponentsBalun 100 Decoupling & Match) 94 C₁ = 75 pf C = 51 pf C1 = 91 pf C₂ = 68pf + 1-16 pf L = 142 nh L1 = 88 nh C₃ = 68 pf + 1-16 pf C2 = 91 pf C₄ =75 pf L2 = 88 nh L3 = 4.7 uh Diodes = UM 9415 PIN Diode C3 = .01 uf C4 =.01 uf C5 = .01 uf C6 = .01 uf Lcomp = 198 nh

FIG. 10 is an electrical schematic diagram for surface coil 55, whichincludes two co-rotating loops 55 a and 55 b. Component values for theelements shown in FIG. 10 are as follows: Network (Correction, LoopComponents Balun 100 Decoupling & Match) 94 C₁ = 43 pf C = 51 pf C1 =102 pf C₂ = 43 pf L = 142 nh L1 = 75 nh C2 = 102 pf L2 = 72 nh L3 = 4.7uh Ccomp = 30 pf + 1-16 pf C3 = 30 pf C4 = 30 pf C5 = .01 uf C6 = .01 ufCtune = 43 pf + 1-16 pf Diodes = UM 9415 PIN Diode

FIG. 11 is an electrical schematic diagram for surface coils 46 and 47.Since the surface coils 46 and 47 may be used in either bilateral mode(both on) or unilateral mode (one on, the other off), each of thesurface coils has a 50 Ohm output 100 and a PIN diode switch, D₁.Component values for the elements shown in FIG. 11 are as follows:Network (Correction, Loop Components Balun Decoupling & Match) 94 C₁ =41 pf C = 51 pf C match 47 = 120 pf C₂ = 51 pf + 1-16 pf L = 142 nh Cblock 47 = 62 pf C₃ = 51 pf + 1-16 pf L block 47 = 142 nh C₄ = 41 pf LISO 47 = 74 nh C match 46 = 91 pf C block 46 = 62 pf L block 46 = 142 nhL ISO 46 = 92 nh Diodes, D₁ = UM 9415 PIN Diode

FIG. 12 is an electrical schematic diagram for surface coils 56 and 57.Like the surface coils 46 and 47, the surface coils 56 and 57 may beused in either bilateral or unilateral mode. A low-pass phase shift andmatching network 102 couples the balun 100 to the network 94 for eachcoil 56 and 57. Component values for the elements shown in FIG. 12 areas follows: Loop Components Balun 100 Low Pass Match 102 C₁ = 56 pf C =51 pf C = 24 pf C₂ = 68 L = 142 nh L = 258 nh C₃ = 75 pf + 1-16 pf C₄ =75 pf + 1-16 pf C₅ = 68 pf C₆ = 56 pf Network (Correction, Decoupling &Match) 94 C1 = 103 pf L1 = 86 nh C2 = 103 pf L2 = 86 nh C3 = 33 pf L3 =198 nh L4 = 198 nh Diodes = UM 9415 PIN Diode

FIG. 13 is an electrical schematic diagram for surface coils 48 and 49.The surface coils 48 and 49 may be used in either bilateral orunilateral mode. Component values for the elements shown in FIG. 13 areas follows: Network (Correction, Loop Components Balun 100 Decoupling &Match) 94 C₁ = 75 pf C = 51 pf C match 49 = 270 pf C₂ = 82 pf L = 142 nhC block 49 = 62 pf C₃ = 75 pf + 1-16 pf L block 49 = 142 nh C₄ = 75 pf +1-16 pf L ISO 49 = 74 nh C₅ = 82 pf C match 48 = 240 pf C₆ = 75 pf Cblock 48 = 62 pf L block 48 = 142 nh L ISO 48 = 92 nh Diodes = UM 9415PIN Diode

FIG. 14 is an electrical schematic diagram for surface coils 58 and 59.The surface coils 58 and 59 may be used in either bilateral orunilateral mode. A low-pass phase shift and matching network 102 couplesthe balun 100 to the network 94 for each coil 58 and 59. Componentvalues for the elements shown in FIG. 14 are as follows: Loop ComponentsBalun 100 Low Pass Matching 102 C₁ = 91 pf C = 51 pf C = 24 pf C₂ = 91pf L = 142 nh L = 258 nh C₃ = 68 pf + 1-16 pf C₄ = 68 pf + 1-16 pf C₅ =91 pf C₆ = 91 pf Network (Correction, Decoupling & Match) 94 C1 = 130 pfL1 = 65 nh C2 = 130 pf L2 = 65 nh C3 = 47 pf L3 = 142 nh L4 = 142 nhDiodes = UM 9415 PIN Diode

FIG. 15 is an electrical schematic diagram for surface coils 50 and 51.The surface coils 50 and 51 may be used in either bilateral orunilateral mode. A low pass phase shift and matching network 102 couplesthe balun 100 to the network 94 for each coil 50 and 51. Componentvalues for the elements shown in FIG. 15 are as follows: Loop ComponentsBalun 100 Low Pass Matching 102 C₁ = 68 pf C= 51 pf C = 24 pf C₂ = 68 pfL= 142 nh L = 258 nh C₃ = 62 pf + 1-16 pf C₄ = 62 pf + 1-16 pf C₅ = 68pf C₆ = 68 pf Network (Correction, Decoupling & Match) 94 C1 = 130 pf L1= 68 nh C2 = 130 pf L2 = 68 nh C3 = 47 pf L3 = 114 nh L4 = 114 nh Diodes= UM 9415 PIN Diode

FIG. 16 is an electrical schematic diagram for surface coils 60 and 61.The surface coils 60 and 61 may be used in either bilateral orunilateral mode. A low-pass phase shift and matching network 102 coupleseach balun 100 to the network 94. Component values for the elementsshown in FIG. 16 are as follows: Loop Components Balun 100 Low PassMatching 102 C₁ = 110 pf C = 51 pf C = 24 pf C₂ = 91 pf L = 142 nh L =258 nh C₃ = 62 pf + 1-16 pf C₄ = 62 pf + 1-16 pf C₅ = 91 pf C₆ = 110 pfNetwork (Correction, Decoupling & Match) 94 C1 = 180 pf L1 = 45 nh C2 =180 pf L2 = 45 nh C3 = 100 pf C4 = 100 pf L3 = 44 nh Diodes = UM 9415Diode

As shown in FIGS. 11 through 16, the surface coil pairs in the lower legportion of the peripheral vascular array 40 include an isolation networkwhich operates to cancel the coupling due to mutual inductance. Whilethe mutual inductance could have alternatively been reduced byoverlapping the adjacent coils in the surface coil pairs (46, 47), (48,49), (50, 51), (56, 57), (58, 59) and (60, 61), the use of the isolationnetwork is preferable because it allows the loops in the coil pairs tobe significantly smaller. As a consequence, the signal-to noise ratio isimproved. In addition, by using smaller separated coils with anisolation network rather than overlapping larger coils, aliasing effectsare reduced. Moreover, the isolation networks allow the surface coilpairs to operate as either a single loop (e.g. in an unilateral mode) oras combined counter-rotating and co-rotating pairs (e.g. in bilateralmode).

The surface coil array 10 described above may be connected to well knownscanners to obtain a variety of images. The coils are typicallyconnected to signal receivers in the scanners via preamplifier inputs.The number of signal receivers in a scanner is preferably kept small dueto the cost of the signal receiver. For example, one known scanner usesfour signal receivers which may receive signals from as many as eightpreamplifier inputs. As discussed above, a preferred surface coil array10 may contain as many as 20 coils 12. The coil interface 20 illustratedin FIG. 1 may be used to select groups of coils from the N surface arraycoils 12(1) through 12(N) to connect to the P preamplifier inputs to Msignal receivers where N is greater than both M and P.

The coil interface 20 in FIG. 1 includes a switch 22 and a logic circuit24 for controlling the state of the switch 22. The logic circuit 24controls the state of the switch 22 according to configurations orgroups of coils 12 that are combined to produce images targetingspecific areas of the body. The configurations may be specified bysignals at a coil select input 28 from the scanner 30 in response touser input. Alternatively, signals may be generated by otheruser-accessible sources, such as dip-switches or other suitable devicesthat may be connected to the interface by a cable, which may be electricor fiber optic. An infrared connection may also be used for remotecontrol selection of coil groups.

FIG. 17 is a block diagram of a scanner 300 and a 20-coil surface coilarray 120 that uses a coil interface 200 according to a preferredembodiment. The coil interface 200 in FIG. 17 includes atransmit/receive (“TR”) driver 130, an RF switch array 220 and controllogic 240, and interfaces the surface coil array 120 to the scanner 300at a pre-amp array 303, which is internal to the scanner 300. Thepre-amp array 303 connects to receivers 301 via a switching and routingcircuit 302. The surface coil array 120 may, for example, be arranged inthe form of the peripheral vascular array 40 described above withreference to FIG. 2A. The scanner 300 in a preferred embodiment is aSigna system with the phased array option from General Electric, asdescribed above.

As shown in FIG. 17, the control logic circuitry 240 includes an RFswitch controller 250, a TR driver controller 260 and error checkingcircuitry 265. The control logic circuitry 240 receives a coil selectinput 270 from the scanner 300 as the coil group selector input. Thecoil select signals at the coil select input are a four-bit digital wordwith DC voltage levels providing binary logic levels. The coil selectinput 270 may be coupled to the lines that connect to the coilsthemselves, and may thereafter, have some RF components. Inductors L₁₋₄filter out any RF components so that a DC signal is received by thecontrol logic circuitry 240.

The control logic circuitry 240 also receives a mode signal from a modeswitch 242. The mode switch 242 allows a user to select a unilateralright, a unilateral left or a bilateral imaging mode. The modes areuseful where right and left coils may be combined in the bilateral modeto obtain an image with a wider field of view, or isolated in the rightor left modes to isolate a selected side. One advantage of isolating aselected side is that an improved signal to noise ratio is obtainedthereby providing an image with a higher resolution.

The RF switch controller 250 uses the coil select input 270 and the modesignal from the switch 242 to select RF switch control lines 280. Theselected RF switch control lines 280 enable RF switches in the RF switcharray 220, which connect selected coils from the surface coil array 120,to couple image signals from the selected coils to the inputs 304 of thescanner 300.

The coil select input 270 is preferably coupled to the TR drivercontroller 260. The TR driver controller 260 uses the coil select input270 to determine which coils are going to be used for imaging. The TRdriver controller 260 outputs signals on the coil enable inputs 261 toenable the coils that are to be used for imaging and disable theremaining coils. The coil select input 270 advantageously permits theuser to select different coil configurations without any scan roomintervention.

The coil select input 270 may, for example, be a four-bit word generatedby the scanner 300 when the user enters a request for images requiring acertain coil configuration. The user's request may be entered at aconsole (not shown). Alternatively, an input that is separate from thescanner 300 may be used. For example, a separate keypad may be used toinput signals that designate a desired coil combination. Other inputsinclude, DIP switches, toggle switches, etc. To enter the request, theuser may enter the four-bit word itself, a group identifier, a requestfor an image of a body part, or any other suitable input that thescanner 300 is programmed to understand as a group of coils or sequenceof coil groups. The four-bit word in a preferred embodiment actually hasthe dual function of communicating a Transmit/Receive state to the coilsas well as providing a group configuration input. When the scanner 300generates a +5 v. signal on all coil select input lines 270, the scanneris in the Transmit state, in which case the remote coil is active andall of the receive coils (i.e. the coils in the surface coil array 120)are preferably actively disabled, such as by the PIN diode switchesshown in FIGS. 5 through 16 for the peripheral vascular array 40, andnot connected to the preamplifiers 303. When not all of the coil selectinput lines 270 are at +5 v., the coils connected to the preamplifiers303 are selected in accordance with the four-bit word.

The logic control circuitry 240 includes an error checking controller265 for sensing error conditions in the coils 120 or coil interface 200.The error checking controller receiving error states from the TR driver130 or error state lines 262, which are described below with referenceto FIG. 19. The error checking controller 265 may also generate faultconditions on transistors 267(1)-267(4) to check for errors. Transistors267(1)-267(4) are normally in a non-conducting state. When switched to aconducting state, the coil select input lines 270 may be put in an errorchecking made by switching the states of the lines to a logic 0 or logic1 to detect a specific fault. Conditions such as coil diode shorts,diode opens, DC power failure and TR driver failure may be sensed onlines 262 in response to the fault generated.

In a preferred coil interface 200, the 20 surface coils are grouped intogroups of coils that produce specific, useful images. FIG. 2Aillustrates the posterior and anterior coils 40 as COIL1 42, COIL2 43,COIL3 52, COIL4 53, COIL5 44, COIL6 54, COIL7 45, COIL8 55, COIL9 46,COIL10 47, COIL11 56, COIL12 57, COIL13 48, COIL14 49, COIL15 58, COIL1659, COIL17 50, COIL18 51, COIL19 60 and COIL20 61. FIG. 18 is a coilgroup table 400 that describes groups of coils 402, a mode switchsetting 404, coils selected for a group at 406, and comments 408describing an image obtained by selecting the group of coils identifiedin each row of the table.

As shown in the coil group table 400, the Group 1 coils COIL1 42, COIL243, COIL3 52 and COIL4 53 are selected in order to obtain an image ofvasculature from the renal arteries to the bifurcation. The function ofthe mode switch 242 (in FIG. 17) is illustrated by comparing Group 5with Groups 8 and 10. In Group 5, the mode switch 242 is set to“Bilateral” as indicated in column 404. The coils selected in Group 5are COIL13 48, COIL14 49, COIL15 58, COIL16 59, COIL17 50, COIL18 51,COIL19 60 and COIL20 61. The signals from these coils are combined inpairs as shown in FIG. 18 to provide an image of both the right and leftfeet. By setting the mode switch 242 to “Right” (in FIG. 17) andselecting COIL13 48, COIL1 58, COIL17 50 and COIL19 60 as shown forGroup 8, images of only the right foot and ankle are provided.

It is to be appreciated by one of ordinary skill in the art that FIGS.2A and 18 illustrate one example of a configuration of surface coilsthat may be used with the coil interface of the present invention. Withchanges to the coil interface that are within the ability of one ofordinary skill in the art, any number of coils may be connected to alimited number of inputs according to functionally defined groups.

Referring to FIG. 17, the coil select input 270 is used by the TR drivercontroller 260 to enable coils that are to receive an image signal andto disable all other coils. The TR driver controller 260 determineswhich coils are to be used according to the group identified by thecontrol select input 270. For each coil to be used, a coil enable signalis output on a corresponding coil enable input 261. The coil enablesignal switches the TR driver 130 to the enable state, which permitscurrent to flow through the PIN diode of the selected coil. The TRdriver 130 maintains coils that do not receive a coil enable signal in adisabled state to prevent noise generated by coils from which an imagesignal is not desired. An advantage of enabling only coils that willreceive image signals and disable all of the coils is that the signal tonoise ratio is improved.

In a preferred embodiment, the TR driver 130 includes a coil driver 132for each coil (COIL1, COIL2, COIL3, COIL4) in the surface coil array 120as shown in FIG. 19. The coils 120(1)-120(4) are shown in FIG. 19 withthe PIN diode used to drive the coil and enable an image RF signal to beinput at the RF switch array 220. The coil drivers 132(a)-(d) arearranged in a totem-pole configuration 134 and supplied by a currentsource 136. In FIG. 19, only four coil drivers 132 are shown in a stack.Any number of coil drivers 132 may be connected in a stack. The numberof coil drivers 132 in a stack is preferably the approximate maximumnumber of coils that can be simultaneously driven by the power supply.

Each coil driver 132 includes a differential switch 140(a) in which thegates of two FETs 142(a), 144(a) of opposite type are driven by the coilenable input 261(a). When the coil enable input 261(a) receives a coildisable signal (logic 1, −15 v.), the first FET 142(a) provides acurrent path 146 for current away from the coil 120(1). When the coilenable input 261(a) receives a coil enable signal (logic 0, 5 v.), thesecond FET 144 provides a current path 147 for current through the coil120(1).

One advantage of using the totem pole configuration shown in FIG. 19 isthat the number of coils that can be driven at one time is maximized.For example, if the PIN diodes in the coils are driven by a −10 v(−v=−10 v.) power supply that can provide up to 800 mA, the power supplymay sag to about −8.5 due to wiring losses. Using the totem poleconfiguration, and assuming about a 0.9 v. drop per diode, 9 diodes maybe simultaneously driven by the single current source 136. If each coildriver 132(a)-(d) and coil diode were to be driven by the power supplyin parallel, four or fewer diodes may be driven simultaneously inparallel. Although power supplies may vary according to the type NMRscanner used, the advantages offered by the totem pole configuration,particularly that of maximizing the number of coils driver are stillavailable.

Another advantage of the totem pole configuration is that error-checkingfunctions may be incorporated into the coil interface by sensing thestate of the voltage levels at selected points in the coil drivers132(a)-(d). In a preferred embodiment, at least four error conditionsmay be sensed: coil diode open, coil diode short, transistor (FET) open,transistor (FET) short.

The error conditions in a preferred embodiment may be sensed bygenerating fault conditions as described above with reference to FIG.17, and by using an upper error switch 145(a)(1) and a lower errorswitch 145(a)(2) each having digital outputs to the logic circuit262(a)(1) and 262(a)(2), respectively. The FET transistor 142(a) and FETtransistor 144(a) must be in opposite states at all times. If outputs262(a)(1) and 262(s)(2) of the upper and lower error switches 145(a)(1),145(a)(2) are in the same state, a diode open or a diode short issensed.

For example, if coil enable input 261(a) has an enable signal, the FETtransistor 144(a) is in the ‘ON’ state thereby providing current to thediode in coil1; and the FET transistor 142(a) is in the ‘OFF’ state.When the FET transistor 144(a) is ‘ON’, the lower error switch 145(a)(2)is ‘ON’ and when the FET transistor 142(a) is ‘OFF’, the upper errorswitch 145(a)(2) is ‘OFF’. If the coil PIN diode for COIL1 is open, thelower error switch 147(a) will remain in the ‘OFF’ state even when anenable signal (5 v) is received. Both 262(a)(1) and 262(a)(2) outputswill be sensed in the low state by the logic circuit 240. In a preferredembodiment, the scanning will be aborted when an error is detected.

A window comparator 151 is used in a preferred embodiment to detecttransistor open or transistor short conditions when outputs Q₀ and Q₁are in opposite states and therefore appear normal. If a transistor(such as 142(a), 144(a), etc.) is open, not enough current is beingdrawn through R_(error) (2.8 ohms). The window comparator 151 willdetect a voltage at 153 that is greater than V_(ul). If a transistor isshorted, too much current will be detected by the window comparator whenthe voltage at 153 is lower than V_(il).

The groups in the coil group table 400 in FIG. 18 may be selected usingthe RF switch array 220 in FIG. 17. FIG. 20 shows an implementation ofthe RF switch array 220. The RF switch array 220 in FIG. 20 includes RFswitches SW1-SW20 and RF combiners CMB1-CMB6 connected in theconfiguration shown. The RF switches SW1-SW20 are enabled by controlinputs 280. Control inputs 280 each include one or more control linesthat are controlled by the logic circuit 240 as described below withreference to FIGS. 23A and 23B.

The configuration of RF switches SW1-SW20 and combiners CMB1-CMB6determines the surface coils to be selected for input of the imagesignal according to the groups selected from the coil selection input270 and mode switch 242 (in FIG. 17). The RF switches SW1-SW20 mayinclude the switches illustrated in FIGS. 21A-21C, as well as variationsof the switches in FIGS. 21A-21C.

The switches in FIGS. 21A-21C use PIN diodes as the preferred switchingelement. PIN diodes are fast, non-magnetic switches that may have aresistance on the order of a few ohms in the ‘on’ state.

FIG. 21A illustrates a single RF switch having one control input 450controlling a single PIN diode D1. The RF switch input 460 is coupled toa coil with an RF imaging signal that may include a DC voltage. When thecontrol input 450 is set to a voltage that is sufficiently positive toforward bias the PIN diode D1, the diode D1 switches to a conductingstate and behaves like a resistor. The diode D1 conducts the RF imagingsignal at the input through capacitors C1 and C2, which block any DCcomponents, to RF switch output 470. The inductors L1 and L2 filter outthe RF signal from the control input 450 and from ground allowing thesignal to be coupled to the output 470.

FIG. 21B illustrates an RF switch having a single input 460 that canswitch to either of two outputs 470 a and 470 b. The RF signal coupledto RF switch input 460 is output to output 470 a when control input 450a forward biases diode D1 and to output 470 b when control output 450 bforward biases diode D2. In one variation of the switch in FIG. 20B,multiple PIN Diodes D1, D2 may share the same control input 450.

FIG. 21C illustrates an RF switch having multiple inputs and a singleoutput. Each input 460 a, 460 b, 460 c couples to a respective diode D1,D2, D3. The diodes D1, D2, D3 are connected to a common output 470. Whenthe control input 450 a, 450 b, or 450 c corresponding to the diode D1,D2 or D3 forward biases the diode, the signal at the input is coupled tothe RF output 470. Multiple PIN Diodes D1, D2 may share the same controlinput 450.

FIGS. 22A-22B are schematic representations of RF switch array 220illustrating the components in RF switches SW1-SW20 and combinersCMB1-CMB6. RF switches SW1, SW2 and SW5 are shown with RF switch controlinputs 450, RF switch inputs 460 and RF switch outputs 470 labeledaccording to the conventions in FIGS. 21A-21C. The RF control inputs 450for each switch interface to the RFS1-RFS33 lines on ports P2 and P4.Ports P2 and P4 in a preferred embodiment interface to the control logic240 which includes circuitry for selecting coils.

FIGS. 22A and 22B illustrate the control of RF switches by selectivelyenabling RFS1-RFS33. For example, if the coil select input designates acoil group, the RF switch controller 250 determines which RF switch orswitcher are to be enabled. The coil enable signal (i.e. logic or +5V ina preferred embodiment) is output by the RF switch controller 266 in thecontrol logic circuitry 240 on RFS03. The 5V signal forward biases diodeD1. With Diode D1 forward biased, the RF signal at coil 8 is output bySW1 at RF switch output 470.

As shown in FIG. 22A, the output 470 of switch SW1 is coupled to bit Aof the coil select input 270. Capacitor C2 blocks the DC voltage signalapplied to the output 470 when the coil select input 270 selects a coilgroup. By blocking DC signals capacitor C2 permits Diode D1 to beforward biased.

The combiners CMB1-CMB6 are typical RF signal combiners such asWilkerson combiners that are used to combine RF imaging signals from twoseparate coils. For example, Groups 4 and 5 in FIG. 6 use signals thatare a combination of RF imaging signals from different coils.

The preamplifiers 303 in the scanner 300 are generally sensitive tosource impedance, which in FIG. 17, for example, is dependent upon theRF electrical characteristics of the coil interface 200 and the surfacecoil array 120. This sensitivity is typically quantified in terms of thenoise figure of the preamplifiers 303.

In accordance with a preferred embodiment of the present invention, theRF design of the coil interface 200 and the surface coil array 120minimizes the effect of this sensitivity by presenting the preamplifiers303 with substantially the same source impedance, regardless of the modeof operation (left, right or bilateral) of surface coil array 120. Thismay be accomplished by setting the electrical length of the entiretransmission path from the surface coil to the preamplifier 250 to beequal to an odd multiple of quarter-wavelengths. Since the combinersCMB1-CMB6 in the RF switch 220 are in the transmission path forbilateral imaging and out of the transmission path for unilateralimaging, the bilateral imaging transmission path includes additionalphase delay from the combiners CMB1-CMB6, which may be compensated forby using a phase advance T network, in series with the combinersCMB1-CMB6. In a similar manner, a π network may be used to adjust theelectrical length of the unilateral imaging transmission path. Theimplementations of T and π phase-shifting networks are well known tothose skilled in the art.

The selection of coils for the input of RF imaging signals isaccomplished by the control logic 240 which uses the coil select input270 and mode select switch 242 (in FIG. 17) to output control signals onthe RFS1-RFS233 lines 280. FIG. 23A illustrates a programmable logicdevice (PLD) U93 used to output control signals RFS1-RFS33 in responseto the coil select input 270 and mode switch 242. The PLD U93 outputscontrol signals at outputs PLD101-PLD133. FIG. 23B is a table thatillustrates the RFSxx signal that corresponds to the PLDxxx signals inFIG. 23A. FIG. 23B also illustrates the states of coil select input 270and the states of mode select switch 242. The states of the RSFxx linesat 280 corresponding to the states of the coil select input 270 and modeselect switch 242 are also provided in FIG. 23B. A state of ‘0’ forRSFxx indicates that the corresponding switch is enabled. The logical‘0’ in a preferred embodiment is set at 5 v. while the logical ‘1’ isset at −15 v. The state of ‘0’ therefore forward biases the PIN diode atthe control input of the switch corresponding to the specified RSFxxline.

FIG. 23C shows the states of the coil enable inputs 261 according to thecoil select input 270 and mode switch 242. FIG. 23C illustrates thecoils selected for various states of the coil select input 270 and themode select switch 242. By referring to FIGS. 23B and 23C, one ofordinary skill in the art can determine the combinations of RF switchesSW1-SW20 and coil enable inputs 261 used to select coils for the desiredcoil groups.

In accordance with a preferred method for imaging the peripheralvasculature with the peripheral vascular array 40, a combination ofcontrast study and time-of-flight imaging is utilized. Generallyspeaking, the use of a contrast agent, such as Gadolinium, will improveimage quality and reduce inspection times. Such contrast agents are,however, relatively expensive and the imaging of the entire peripheralvasculature would require a substantial amount of the contrast agent.The method therefore utilizes a contrast agent for imaging only thoseareas where time-of-flight imaging is difficult.

In particular, the method includes the step of performing a contraststudy of the renal arteries and the abdominal bifurcation by acquiringimage information using surface coils 42, 43, 52 and 53. The timing ofimage acquisition is coordinated with the injection of the contrastagent in any known manner. Time-of-flight imaging is then utilized toacquire image information from the vasculature in the legs, using, forexample, surface coils 44 through 49 and 54 through 59. Images of thefeet may be obtained using either the contrast study or thetime-of-flight technique by acquiring image information from surfacecoils 50, 51, 60 and 61.

In the alternative, images of the peripheral vasculature may be obtainedusing the peripheral vascular array 40 with only time-of-flight imaging.This technique, however, may require longer examination times due to thedifficulty of using time-of-flight imaging to acquire image informationin structures having sagittal plane blood flow, such as the renalarteries.

In accordance with another preferred method, the peripheral vasculararray 40 acquires successive adjacent axial images in timed relation tothe progression of a bolus of contrast agent through the peripheralvasculature. This is made possible by the large area covered by theperipheral vascular array 40, which allows images from the renalarteries through the feet to be obtained without repositioning the array40.

While the invention has been described in conjunction with presentlypreferred embodiments of the invention, persons of ordinary skill in theart will appreciate that variations may be made without departure fromthe scope and spirit of the invention. The true scope and spirit of theinvention is defined by the appended claims, interpreted in light of theforegoing description.

1. A coil interface in an imaging system, comprising, in combination: Nimage signal inputs; a plurality of coil switches connected to aplurality of said N image signal inputs; circuitry for selecting a groupof said N image signal inputs by enabling a selected group of saidplurality of switches in response to a group selector input; and M imagesignal outputs, at least one of said image signal outputs beingconnected to a selected group of N image signal inputs when saidselected group of said plurality of switches is enabled, said imagesignal outputs being operable to receive said image signals when N isgreater than M.
 2. The coil interface of claim 1 further comprising ppreamplifiers, at least one of which is operatively connected at apreamplifier input to said selected group of N image signal inputs whensaid selected group of said plurality of coil switches is selected, saidpreamplifiers having preamplifier outputs connected to correspondingimage signal receivers, wherein p is greater than or equal to M.
 3. Thecoil interface of claim 2 further comprising: a plurality ofpreamplifier switches, each of said preamplifier switches having atleast one signal input connected via selected coil switches to said Nimage signal inputs and signal outputs connected to correspondingpreamplifiers.
 4. The coil interface of claim 1 wherein said pluralityof coil switches includes an RF switch comprising: at least one RFswitch input each coupled to a corresponding one of said N image signalinputs; and at least one PIN diode corresponding to said at least one RFswitch input, said at least one PIN diode coupled at the anode to thecorresponding one of said at least one RF switch input and coupled atthe cathode to an RF switch output, said PIN diodes operative to outputthe image signal from said one of said N image signal inputs in aforward bias state and operative to block said RF signal in a reversebias state.
 5. The coil interface of claim 4 further comprising at leastone control input coupled to a corresponding one of said at least onePIN diode to set said PIN diode in the forward bias state when saidcontrol input receives an enable signal and in the reverse bias statewhen said control input receives a disable signal.
 6. The coil interfaceof claim 1 wherein said plurality of coil switches includes an RF switchcomprising: an RF switch input coupled to a corresponding one of said Nimage signal inputs; and a plurality of PIN diodes coupled at thecathode of each PIN diode to a corresponding plurality of RF switchoutputs and coupled at the anode to the RF switch input, said PIN diodesoperative to output the image signal from said one of said N imagesignal inputs in a forward bias state and operative to block said RFsignal in a reverse bias state.
 7. The coil interface of claim 6 furthercomprising a plurality of control inputs coupled to set a correspondingPIN diode in the forward bias state when said control input receives anenable signal and in the reverse bias state when said control inputreceives a disable signal.
 8. The coil interface of claim 1 wherein saidplurality of coil switches includes an RF switch comprising: a pluralityof RF switch inputs each coupled to a corresponding one of said N imagesignal inputs; and a plurality of PIN diodes coupled at the cathode ofeach PIN diode to a corresponding plurality of RF switch outputs andcoupled at the anode to a corresponding one of the plurality of RFswitch inputs, said PIN diodes operative to output the image signal fromsaid one of said N image signal inputs in a forward bias state andoperative to block said RF signal in a reverse bias state.
 9. The coilinterface of claim 8 further comprising a plurality of control inputscoupled to set a corresponding PIN diode in the forward bias state whensaid control input receives an enable signal and in the reverse biasstate when said control input receives a disable signal.
 10. The coilinterface of claim 1 further comprising at least one RF combiner forcombining the image signal from one of said N image signal inputs withthe image signal from another one of said N image signal inputs.
 11. Aninterface for connecting a surface coil array to an NMR scanner,comprising: a first number, N, of RF inputs, the RF inputs beingarranged to receive signals from the surface coil array; an RF switchcircuit coupled to the RF inputs, said RF switch circuit having a secondnumber, M, of RF outputs, wherein the RF switch circuit comprises afirst plurality of RF switches coupled to the RF inputs, a secondplurality of RF combiners coupled to receive outputs from a subset ofthe first plurality of RF switches, and a third plurality of RF switchescoupled to receive outputs from the second plurality of RF combiners;and a control logic circuit coupled to the RF switch circuit, whereinthe control logic circuit receives a coil select signal and controls astate of the first plurality of RF switches and the third plurality ofRF switches in response to the coil select signal.
 12. An interface asclaimed in claim 11, wherein the coil select signal is provided by theNMR scanner.
 13. An interface as claimed in claim 11, wherein the firstnumber, N, is greater than the second number, M.
 14. An interface asclaimed in claim 13, wherein N=20 and M=8.
 15. An interface as claimedin claim 11, wherein the subset of the first plurality of RF switcheseach have one input coupled to the RF inputs and two outputs, wherein asignal at the RF switch input is coupled to one of the two outputs inaccordance with a signal from the control logic circuit.
 16. Aninterface as claimed in claim 15, wherein one of the two outputs of theRF switch is coupled to one of the second plurality of RF combiners andthe other output of the RF switch is coupled to an input to one of thethird plurality of RF switches.
 17. An interface as claimed in claim 11,wherein each RF combiner in the second plurality of RF combiners isoperable to combine a pair of RF inputs from the surface coil array intoa single RF output.
 18. An interface as claimed in claim 17, wherein thesingle RF output is coupled by one of the third plurality of RF switchesto the NMR scanner.
 19. An interface as claimed in claim 18, whereinfour pairs of RF inputs are combined into four outputs by four RFcombiners, the four outputs being coupled to the NMR scanner.
 20. Aninterface as claimed in claim 11, further comprising a transmit/receivebias circuit coupled between the RF inputs and the RF switch, whereinthe transmit/receive bias circuit is coupled to and controlled by thecontrol logic circuit to actively disable selected coils from thesurface coil array.